Hey, uh, have you heard  there’s an eclipse coming up?

A total solar eclipse will be visible over  this part of the United States on April 8, and we know it’ll be the last one we can  see in North America until the mid-2040s. But can you imagine what it  would have been like for people a couple thousand years ago  when things like this happened?

It must have been terrifying to be hanging out and just see the sun go out, with no warning. Except…it wasn’t. Because humans have been predicting  eclipses since the 7th century BCE.

As it turns out, our ancestors  figured out how to do that without calculators or even  telescopes, and here’s how. [♪ INTRO] If you need a refresher, an  eclipse is when a celestial body blocks the light of another celestial body. So for instance, a lunar eclipse is  when the earth passes right between the Sun and the moon, which blocks the  sunlight from bouncing off the moon and causes the moon to go dark. While lunar eclipses are relatively  common, total solar eclipses are rarer.

For these to occur, the moon has  to be the monkey in the middle and block the sun from an  observer’s viewpoint on Earth. You can imagine that the first  couple times that this happened where people could see it, those  people would want to understand why. And the story of figuring out that  why begins roughly 2,600 years ago, in the Mesopotamian city-state called Babylon.

The full history of the ancient Babylonians  is vast and spans over a millennium. But for our purposes today, the main thing to know is that they were really interested  in the study of astronomy. Records from early Babylon  in the form of clay tablets show how obsessed they were with the stars.

For instance, they documented  the movement of constellations that served as the origin of  what we know today as the Zodiac. And they were the first group  of people we have records of to make mathematical  predictions of celestial events. Their ability to track the  movements of the sun and moon let them develop a sophisticated calendar.

Like, some of those ancient clay tablets  are the earliest recorded evidence we have of people making mathematical predictions of celestial bodies’ movements, ever. And for predicting eclipses, they  worked out a complicated system called Saros cycles, that are still  used today when referring to eclipses. A Saros cycle is a period of 223 synodic months.

A synodic month is the period of time  it takes the moon to complete one cycle, so for example, from one new moon to the next. The moon’s cycle doesn’t divide evenly  into solar days, so a synodic month is roughly 29 days, 12 hours, 44 minutes  and 3 seconds – about 29 and a half days. And the 223 synodic months  that make up one Saros cycle translates to just over 18 years.

Fortunately for us, the  ancient Babylonians showed us exactly how they figured this out, too. A clay tablet called BM 45861 has all the details of how they calculated the Saros cycles. The Babylonians observed  that once you saw an eclipse, 223 synodic months later the moon  would have returned to the same plac relative to the sun and the Earth, and  you could expect to see another eclipse.

The geometry of how the celestial bodies are all   moving means that the alignment only lasts  for roughly between 1200 and 1500 years. Which is obviously a long time to us, but  it’s a blink of an eye in terms of space. But even though they do repeat, none  of the Saros cycles last forever.

And yes, that is plural. There’s more than one Saros cycle, and  multiple cycles are always happening at the same time, because there’s  more than just one way to line up the Sun and moon and Earth that causes an eclipse. NASA has records of 180 total Saros  cycles, which includes ones that have ended a long time ago and their predictions  of ones that haven’t even started yet.

Having this many cycles makes sense, because we don’t only see eclipses every 18 years. In North America, our last  solar eclipse was in 2017 and that one was part of cycle 145. Meanwhile, the 2024 eclipse  will be part of cycle 139.

The period of 18 years is when  the next eclipse sharing that same celestial geometry is going to occur,  not just the next eclipse at all. And eclipses in one Saros cycle aren’t visible from that exact same spot 18 years later. The path of totality, AKA  where you get the best view, shifts about 120 degrees West  for each eclipse in the series.

Why? Because the Saros cycle is 18  years, 11 days, and 8 hours long. So the part of the planet experiencing  daylight and thus, facing the sun, changes with each eclipse in that series.

So yeah, this is pretty complicated. Which makes it even more impressive  that Babylonians figured it all out! Early astronomers were so  good at recording their data that we have evidence of an eclipse  that occurred around 1223 BCE from the region now located in modern Syria.

Unfortunately we don’t know for sure when  it happened, because translating dates from the ancient Syrian calendar into  the modern Gregorian one is… difficult. But even with that fuzziness, we’re talking about something  almost 3000 years ago. And the really interesting thing is that  historians think that the solar eclipses may not have been all that important to  the Babylonians, at least not at first.

We think that the Babylonian scholars  tracked lunar eclipses first, because those occur way more  often than solar eclipses. But the thing is, once you figure  out the math for lunar eclipses, you can apply it to the solar eclipses too. So what did the Babylonians do with  their eclipse-predicting powers?

Because they probably weren’t selling  merch – but more on that later. We have evidence that people in lots of societies viewed the eclipses as bad  omens, which, I mean, I get it. And eclipses of either variety were  supposed to be particularly bad luck for the king, at least to the Mesopotamian  descendants of the Babylonians.

So in order to protect their monarch,  they would appoint a substitute, decoy king to serve during  the hours of the eclipse. This way, the bad luck was believed to fall  on the substitute instead of the real guy. They’d even assign the fake  king a fake queen, too.

And what do you do with a decoy  king after the sun is back on? Well, you sacrifice him, and his fake queen too. So those two might not have been  all that thrilled to know when an eclipse was coming, even if everyone else was.

So anyway, it’s clear that we’ve been  curious about eclipses for a long time, and we owe a lot of our progress in astronomy to the ancient Babylonians who started us off. Which is something I am grateful for, even if those sacrificed decoy royals  probably didn’t feel the same way. If this episode got you hyped up for the  eclipse in April, you should know that in order to look right at it, you’re  gonna need some protective eyewear.

And while you’re at it, why not  pair your new specs with our awesome Path of Totality band-style t-shirt? Celebrate this stellar solar  phenomenon in style, baby! Links to everything… down below!

Happy eclipse-watching everybody! [♪ OUTRO]